Refrigeration process for use in liquefication of gases

ABSTRACT

A refrigeration process for use in the liquefication of gases wherein a gaseous mixed refrigerant is compressed to form a mixture of a gas difficult to condense and a liquid, at least a portion of a liquid, after reduction to a lower pressure is used to effect cooling and condensation of substantially all of the gas, the condensed gas after reduction to lower pressure is employed as a low temperature refrigerant in the cold end of an exchanger and the remaining portion of said liquid is employed as a high temperature refrigerant in an intermediate stage of said exchanger. The process is used to supply the mechanical refrigeration needed in a turboexpansion system for the liquefication of natural gas and the like.

United States Patent Swearingen et al.

[451 Sept. 12, 1972 [54] REFRIGERATION PROCESS FOR USE IN LIQUEFICATION OF GASES [72] Inventors: Judson S. Swearingen, 500 Bel Air Rd., Los Angeles, Calif. 90024; Sham Sunder, 2235 Carmelina Ave., Los Angeles, Calif. 90064 22 Filed: Nov. 17, 1969 21 Appl. No.: 877,116

[52] US. Cl. ..62/40, 62/9, 62/ 1 1, 62/38 [51] Int. Cl. ..F25j 1/00, F25j 1/02 [58] Field of Search ..62/23, 24, 26, 27, 28, 30, 62/38, 39, 40; 55/512, 55

[56] References Cited UNITED STATES PATENTS V 3,348,384 10/1967 Harmens .....62/26 3,548 ,606 12/1970 Kuerston ..62/40 3,581,510 11/1965 Hughes ..62/40 3,218,816 11/1965 Grenier ..62/26 3,274,787 9/1966 Grenier ..62/23 3,343 ,374 9/1967 Nelson ..62/40 FROM COMPRESSOR 3,364,685 l/l968 Perret ..62/40 3,559,373 2/1971 Garrett ..62/1 1 3,203,191 8/1965 French ..62/38 Primary Examiner-Norman Yudkoff Assistant ExaminerArthur F. Purcell AttorneyRalph R. Browning 57 ABSTRACT A refrigeration process for use in the liquefication of gases wherein a gaseous mixed refrigerant is compressed to form a mixture of a gas difiicult to condense and a liquid, at least a portion of a liquid, after reduction to a lower pressure is used to effect cooling and condensation of substantially all of the gas, the condensed gas after reduction to lower pressure is employed as a low temperature refrigerant in the cold end of an exchanger and the remaining portion of said liquid is employed as a high temperature refrigerant in an intermediate stage of said exchanger. The process is used to supply the mechanical refrigeration needed in a turboexpansion system for the liquefication of natural gas and the like.

5 Claims, 2 Drawing Figures TO TURBO EX PANDE R 1 l M- X547 EXPANSION 29/ wan 5 V \40 SHEET 1 0F 2 PATENTEBsEP 12 m2 4 ow mN NW R Sham Sunder Judson S. Swearingen IN VENTOR ATTORNEY tmqzqixw OQWSH E QQWWMWEZQU 26ml REFRIGERATION PROCESS FOR USE IN LIQUEFICATION OF GASES BACKGROUND OF THE INVENTION The present invention relates to the liquefication of gases. More particularly, the present invention relates to a refrigeration process for use in the liquefication of gases, especially natural gas.

The so-called cascade method of refrigeration is widely known and used as a refrigeration method in plants liquefying natural gas. Briefly, the method involves liquefying a first refrigerating fluid under pressure, expanding it to a lower pressure and vaporizing it in heat exchange with'a second more volatile refrigerating fluid under pressure, expanding the second refrigerating fluid at a lower pressure and then vaporizing it in heat exchange with a third more volatile refrigerating fluid, and soon, each of the successive refrigerating fluids enabling a lower temperature to be reached. Unfortunately, this process requires the use of equipment which is relatively complicated and which presents great regulation and control difficulties, each of the refrigerating fluids traveling through its own closed loop and requiring a compressor, a condenser, an expansion valve and a heat exchanger.

Plants employing expansion processes and in particular those employing turboexpanders as the expansion engine are meeting with increasing favor in natural gas liquefication installations because of their simplicity, ease of operation, rangeability and relatively low investment cost. In order for these expansion processes to operate, they require, among other things, some mechanical refrigeration at moderate temperatures where the Carnot efficiency of the expander system would be penalized. Such mechanical refrigeration must be efficient, relatively inexpensive and troublefree in order to make the use of turboexpanders economical.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a refrigeration process.

It is another object of the present invention to provide a refrigeration process for use in the liquefication of gases, particularly natural gas.

Yet another object of the present invention is to provide a single refrigeration system having an adjustable rate of refrigeration at relatively high temperature levels and another adjustable rate of refrigeration at relatively low temperature levels.

Still another object of the present invention is to provide a single mechanical refrigeration system for use in a natural gas liquefying plant employing turboexpanders.

These and other objects of the present invention which will become apparent from the drawings, the description given herein and the appended claims are accomplished in one respect by a refrigeration process comprising introducing a gaseous refrigerant mixture into a compression zone to form a first liquid fraction and a first gas fraction, separating the first gas fraction from the first liquid fraction and subsequently condensing substantially all of said first gas fraction in a cooling zone, expanding said first liquid fraction to a lower pressure and forming a second gas fraction and a second liquid fraction, separating said second gas fraction from said second liquid fraction and returning said second gas fraction to said compression zone, expand ing said second liquid fraction to a lower pressure, passing at least a portion of the expanded second liquid fraction through said cooling zone counterflow to said first gas fraction passing through said cooling zone and returning said portion of said expanded second liquid fraction, now vaporized, to said compression zone, returning any remaining portion of said expanded second liquid fraction to said compression zone, expanding the condensed first gas fraction to a lower pressure and returning the expanded condensed first gas fraction after vaporization to said compression zone.

In another embodiment, the present invention provides in a process for the liquefication of a gas wherein a gaseous feed is compressed and cooled to a temperature of at least 60F, at least a portion of the compressed and cooled gaseous feed is passed through a turboexpansion engine, the liquid from the discharge of said turboexpansion engine is flash cooled at'least once to obtain a colder gas, the gaseous fraction from the flash cooling being used to augment cooling of the compressed gaseous feed, an improvement wherein said compressed gaseous feed is cooled by the refrigeration process described above.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic flow sheet of the refrigeration process of the present invention.

FIG. 2 is a schematic flow sheet of a turboexpansion cycle for liquefying a gas incorporating the refrigeration process of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Reference is now made to FIG. 2 for a detailed description of a system for the liquefication of gases employing a turboexpansion engine. For purposes of illustration, the system is described with particular reference to the liquefication of natural gas and more particularly methane. But it is to be understood that the system can be applied to the liquefication of other gases. The mechanical refrigeration shown in the dotted box of FIG. 2, is the refrigeration process set forth in FIG. 1 hereinafter described in detail. The pressures and temperatures of the streams at various points in the process are shown on FIG. 2.

Referring, then, to FIG. 2, methane via line 48 enters compressor 49 which is driven by motor 50. The compressed methane exits compressor 49 via line 51 and passes into exchanger 52 where it is cooled. A portion of the cooled methane exits via line 54 and enters turboexpander 55 by which it is further cooled and a portion condensed, exiting expander 55 via line 56 and passing into separator 57 where the vapor and liquid are disengaged, the vapor portion passing overhead via line 58 and back through exchanger 52. It leaves the exchanger via line 59 and is compressed in compressor 60 driven by the shaft 61 from turboexpander S5, returning to main compressor 49 via line 62. A second portion of the compressed methane after further cooling and condensation exits exchanger 52 via line 64 and is combined with the liquid from the separator 57 both passing into a second flash evaporator 65 where the liquid is further cooled by flash evaporation of a portion thereof. The vapor from flash evaporator 65 passes overhead via line 66 back through coil 73 of exchanger 52 a portion being employed as fuel gas and exiting via line 74. The remaining portion passes via line 75 back into main compressor 49. The liquid from flash evaporator 65 is sent to storage tank 68 for use as desired. Any vapor from storage tank 68 due to some flash vaporization of stream 67 as a result of the latters reduction in pressure and from evaporation in the storage tankis taken overhead via line 69, compressed in compressor 70, driven by motor 71 and is combined via line 72 with the vapor coming from separator 65 via line 66, the combined streams passing through exchanger 52 as described above. While in the gas liquefication cycle described above, two stages of flash cooling are shown, it is to be understood that more than two stages may be employed or alternately that a single stage of flash cooling, while insufficient, may also be employed.

Reference is now made to FIG. 1 for a detailed description of the refrigeration process of the present invention, referred to as the mechanical refrigeration in FIG. 2. The stream of gas to be cooled enters heat exchanger via line 11, is cooled by evaporator coil 13 and exits at a reduced temperature through line 12. The mixed gaseous refrigerant exiting from coil 13 through line 15 is fed into the first stage inlet of compressor 16 where it, along with other streams hereinafter described, is compressed through several stages of compression and is discharged via line 17 into condenser 18 where the less volatile portion of the mixture is condensed.

The partially condensed mixture of refrigerant is discharged into separator 19 where the uncondensed vapor is separated from the liquid fraction, the former exiting separator 19 via line 20, the latter exiting through expansion valve 21 into line 22. On passing through expansion valve 21, the liquid fraction from separator 19 has its pressure reduced whereupon a portion of the liquid flash vaporizes. This vapor-liquid mixture is disengaged in separator 23, the uncondensed vapor portion passing via line 24 into the last stage of compressor 16, the condensed liquid portion being discharged through expansion valve 25 into line 26. Upon passing through expansion valve 25, the liquid fraction from separator 23 has its pressure reduced, whereupon a portion is flash vaporized. This gas-liquid mixture exiting valve 25 is partially separated into a liquid-rich and gas-rich fraction by a centrifugal separation process, which as shown, is accomplished by passing the gas-liquid mixture from valve 25 through curved section 27 of line 26, the heavier liquid being thrown to the outside and exiting through line 28, the gas-rich portion passing through expansion valve 30 through line 31 into exchanger 32 and out of exchanger 32 via line 33 back to compressor 16. The gas-rich fraction upon passing through the shell of exchanger 32 counterflow to the uncondensed gas fraction from separator 19 fully vaporizes; whereupon it cools and causes complete condensation of the latter. The liquidrich fraction obtained from the partial centrifugal separation of the gas-liquid mixture from expansion valve 25 is passed via line 28, expansion valve 29 and line 14 into an intermediate zone of coil 13 of exchanger 10 where it serves as the higher temperature refrigerant in exchanger 10.

The completely condensed liquid fraction passing through exchanger 32 from separator 19 is expanded through valve 35 and can then take one of two routes. If the temperature in the cold end of exchanger 10 is colder than is required, the liquid from condenser coil 34 of exchanger 32 upon passing through valve 35 is sent to separator 37 through three-way valve 36 whereupon the vapor portion is passed via line 38 back through the shell of exchanger 32 counterflow to the gas passing via line 20 through coil 34 of exchanger 32. The liquid fraction in separator 37 is passed via line 39 expansion valve 40 and line 41 to the cold end of exchanger 10 where it acts as the low temperature refrigerant. However, if there is difficulty in attaining the low temperature desired in the cold end of exchanger 10, then three-way valve 36 is adjusted such that the entire flow of fluid exiting through valve 35 bypasses separator 37 and passes via line 42 into line 39 and thence through expansion valve 40 and line 41 into the cold end of exchanger 10. Thus it is seen that separator 37 serves the purpose of an economizer and saves power when the temperature in the cold end of exchanger 10 is lower than required.

As will be readily recognized, the quantity of the low temperature refrigerant in the low temperature end of exchanger 10 is determined by valve 40. However, if valve 40 is open too widely such that too much lowtemperature refrigerant is flowing into the cold end of exchanger 10, then expansion valve 35 will serve to act as an over-ride or check. This is accomplished by the fact that valve 35 is controlled by temperature controller and sensor 43 which reads the temperature at an intermediate zone of heat exchanger 32. If too much refrigerant is withdrawn from condenser coil 34 of exchanger 32, there will not be enough refrigerant entering the shell of exchanger 32 from line 35 to completely condense the former with the result that the temperature in the intermediate zone of exchanger 32 will rise perceptively. This temperature rise will be sensed by controller 43 which will then automatically adjust valve 35 to reduce the flow therethrough. On the other hand, if the temperature in the cold end of exchanger 10 is such that more refrigerant is available at valve 40 than is required, there will be a reduced flow in line 20 with the result that the discharge pressure from compressor 16 will increase reducing the flow through compressor 16 and also the power required to drive it.

The moderate to high temperature requirement in exchanger 10 are determined by the setting of valve 29. For example if the refrigeration requirement throughout exchanger 10 is low, a reduced flow will then be passing through valves 40 and 29 resulting in an excessive flow through line 31 into the shell side of exchanger 32 with the result that the stream in line 33 leaving exchanger 32 will have its temperature reduced and in the extreme case will be reduced to the extent that there will be unvaporized liquid in line 33.

In order to optimize the process, temperature sensor 44 is provided in line 33 to detect the temperature of the stream returning therethrough to compressor 16. Depending on the temperature sensed by sensor 44, the speed of compressor 16 is adjusted such that if a temperature low enough to indicate liquid in line 33 is detected, the speed of compressor 16 is accordingly reduced insuring that only the output from compressor 16 absolutely needed is produced. As can be seen, the stream passing in line 33 can take two routes back to compressor 16. If, for example, it is desired to reduce the temperature in coil 13 of exchanger 10, the stream in line 33 passes through three-way valve 45 and is then fed to the first stage of compressor 16 through line 46 thereby reducing the pressure in the shell of exchanger 32 thereby condensing more of vapor stream 20. In this case, the flow through expansion valve 40 is increased. Alternately, the stream in line 33 may pass through three-way valve 45 and line 47 into an intermediate stage of compressor 16 if the low temperature requirements of exchanger are being met.

The advantages of the above described refrigeration process are best demonstrated by reference to the table below which gives the temperatures and pressures at various points in the system shown in FIG. 1. Refrigerant A consisted of a mixture of 35 percent ethylene, 35 percent propane, 20 percent n-butane and 10 percent n-pentane. Refrigerant B consisted of a mixture of percent ethylene, 50 percent propane and 25 percent n-butane.

Refrigerant Pressure and Temperature Table As can be seen from an examination of the data in the above table, the refrigeration process described herein provides an efiicient and simple means for supplying mechanical refrigeration in a plant for liquefying natural gas wherein a turboexpander is used as the expansion engine. It generally preferable in such plants that the inlet temperature to the turboexpander be in the order of 60F. However, the exact temperature depends on numerous valuables and optimum operation may dictate temperatures considerably higher.

The refrigerant employed will be a mixed type containing several components which though not necessarily hydrocarbons, preferably will condense to miscible liquids, and a portion of which condenses at ordinary cooling water temperatures and the remainder at temperatures substantiallylower than this.

The liquid-rich fraction centrifugally separated out of the expanded fraction passing through valve 25 is shown as being sent through a single expansion valve and then to an intermediate temperature zone of exchanger 10. It is to be understood however that the liquid-rich fraction can be divided into a plurality of streams, each stream passing through an expansion valve and each entering the exchanger at different points.

As will be readily recognized by those skilled in the art, numerous modifications of the above described process can be made without departing from either the spirit or scope thereof.

We claim:

1. In a process for the liquefication of a gas wherein a gaseous feed is compressed and cooled to a temperature of at least F., at least a portion of the compressed and cooled gaseous feed is passed through a turboexpansion engine, the liquid from the discharge of said turboexpansion engine is flash cooled at least once to obtain a liquefied gas, the gaseous fractions from the flash cooling being used to augment cooling of the compressed gaseous feed, a portion of said gaseous feed after passing through said turboexpansion engine being recycled and compressed with the fresh gaseous feed, the improvement wherein the compressed gaseous feed is cooled by a refrigeration process comprismg:

A. introducing a gaseous refrigerant mixture in a compression zone to form a first liquid fraction and a first gas fraction;

B. separating said first gas fraction from said first liquid fraction and subsequently condensing in a single step substantially all of said first gas fraction in a cooling zone;

C. expanding said first liquid fraction to lower pressure and forming a second gas fraction and a second liquid fraction;

D. separating said second gas fraction from said second liquid fraction and returning said second gas fraction to said compression zone;

E. expanding said second liquid fraction to a lower pressure;

F. passing at least a portion of the expanded second liquid fraction through said cooling zone and returning said portion to said compression zone, said portion of said expanded second liquid fraction passing through said cooling zone counterflow to said first gas fraction passing through said coolmg zone;

G. returning any remaining portion of said expanded second liquid fraction to said compression zone;

B. expanding the condensed first gas fraction to a lower pressure; and

I. returning the expanded condensed first gas fraction to said compression zone, said remaining portion of said expanded second liquid fraction and said expanded condensed first gas fraction being used to effect cooling of said compressed gaseous feed.

2. The process of claim 1 wherein said discharge from said turboexpansion engine is flash cooled in mul- 0 tiple stages.

3. The process of claim 2 wherein a minor portion of said compressed and cooled gaseous feed is further cooled and condensed, the condensed portion is combined with the liquid from the first flash cooling stage, and the combined condensed portion and liquid from said first cooling stage are passed to said second cooling stage.

4. The process of claim 1 wherein said gaseous feed comprises natural gas.

' 5. The process of claim 1 wherein the compressed gaseous feed is cooled to a temperature of around 60 F. S 

1. In a process for the liquefication of a gas wherein a gaseous feed is compressed and cooled to a temperature of at least -60* F., at least a portion of the compressed and cooled gaseous feed is passed through a turboexpansion engine, the liquid from the discharge of said turboexpansion engine is flash cooled at least once to obtain a liquefied gas, the gaseous fractions from the flash cooling being used to augment cooling of the compressed gaseous feed, a portion of said gaseous feed after passing through said turboexpansion engine being recycled and compressed with the fresh gaseous feed, the improvement wherein the compressed gaseous feed is cooled by a refrigeration process comprising: A. introducing a gaseous refrigerant mixture in a compression zone to form a first liquid fraction and a first gas fraction; B. separating said first gas fraction from said first liquid fraction and subsequently condensing in a single step substantially all of said first gas fraction in a cooling zone; C. expanding said first liquid fraction to lower pressure and forming a second gas fraction and a second liquid fraction; D. separating said second gas fraction from said second liquid fraction and returning said second gas fraction to said compression zone; E. expanding said second liquid fraction to a lower pressure; F. passing at least a portion of the expanded second liquid fraction through said cooling zone and reTurning said portion to said compression zone, said portion of said expanded second liquid fraction passing through said cooling zone counterflow to said first gas fraction passing through said cooling zone; G. returning any remaining portion of said expanded second liquid fraction to said compression zone; H. expanding the condensed first gas fraction to a lower pressure; and I. returning the expanded condensed first gas fraction to said compression zone, said remaining portion of said expanded second liquid fraction and said expanded condensed first gas fraction being used to effect cooling of said compressed gaseous feed.
 2. The process of claim 1 wherein said discharge from said turboexpansion engine is flash cooled in multiple stages.
 3. The process of claim 2 wherein a minor portion of said compressed and cooled gaseous feed is further cooled and condensed, the condensed portion is combined with the liquid from the first flash cooling stage, and the combined condensed portion and liquid from said first cooling stage are passed to said second cooling stage.
 4. The process of claim 1 wherein said gaseous feed comprises natural gas.
 5. The process of claim 1 wherein the compressed gaseous feed is cooled to a temperature of around -60* F. 